Electro mechanical apparatus for monitoring the operation of sucker rod type well pumping units is known to those skilled in the art as evidenced by my previous Pat. Nos. 3,851,995; 4,363, 605; 4,043,191; and 4,208,665. In my Pat. No. 4,363,605, a pumpjack unit has a rod string which is weighed continuously during the pumping action and the resultant data utilized to shut-in the well when a predetermined reduction in weight is sensed. Also note Gibbs Pat. No. 3,951,209 for similar art.
Montgomery Pat. No. 3,817,094 weighs the deflection of the walking beam of a pumpjack unit for providing a signal used for controlling the motor of a pumpjack unit.
In my co-pending patent application Ser. No. 634,544 filed July 25, 1984, relative movement between co-acting components of a pumpjack unit is utilized as a control signal for shutting in a well.
Montgomery Pat. No. 3,838,597 measures the load during the pumping action and produces a signal which shuts-in the well upon encountering a pump-off condition.
Gibbs Pat. No. 4,490,094 measures the instantaneous motor speeds of revolutions for a pumpjack unit, and compares the results with the instantaneous speeds of revolutions of a pump-off condition in order to shut-in a pumpjack unit.
The present invention provides a pump-off control that indirectly measures the efficiency of the pumping action by counting the length of time required for the pumpjack unit to make one complete cycle of operation, or at least a portion of the downstroke; and, when the measured time interval changes a predetermined amount, the well is shut-in for a predetermined length of time. The portion of the measured pumping cycle must include that part of the downstroke where fluid pounding historically occurs.
A pumpjack unit utilizing a string of sucker rods for actuating a downhole pump requires a finite amount of work in order to lift a full barrel of liquid to the surface of the ground each pumping cycle. Most pumpjack units utilize a high slip electric motor which is designed to operate under varying load conditions. All electrical motors slip; that is, the rpm will decrease as the load increases. Pumpjack units utilizing electrical motors have special designed high slip motors which can tolerate the variable load occasioned by the varying power requirement as the pumpjack unit upstrokes and then downstrokes. In other words, the motor is designed to slip a large amount as the load is increased.
The load on the sucker rod can be measured during each pumping cycle, and the resultant data used to plot a graph of the instantaneous load versus the pump plunger position. This plot of data is called a "dynamometer card". The curve of a dynamometer card is drawn by attaching weight measuring and rod position indicator apparatus to the polish rod or bridle of a pumpjack unit, such as discussed in several of my previously mentioned patents for example.
The area defined by the dynamometer curve can be related to horsepower requirement. The horsepower requirement between a full barrel pumping condition, and a partially full barrel pumping condition is considerable. This change in power requirement is reflected in the load required by the high slip motor, which varies considerably between these two extremes. Accordingly, as the pumping action proceeds from a full barrel to a pump-off condition, the time differential required for the pump to downstroke is considerable and this change in the time interval can be utilized as a control signal for detecting a pump-off condition and thereby provide a control for the pumpjack unit.
The time differential always occurs on the downstroke because the hydrostatic head of the column of fluid being supported by the rod string is theoretically removed form the pump plunger during the downstroke, and it is during that interval of time that fluid pounding occurs. Therefore, it is the downstroke of the pumping cycle that varies in time, whereas the upstroke is of a relatively constant time interval.
A comprehension of the above observations is necessary in order to fully appreciate this invention. This novel and unexpected method of generating a signal related to a pump-off condition brings about other patentable concepts. Applicant has observed that the time interval for a full stroke to be carried out on pumpjack unit never exceeds a maximum value of predetermined magnitude unless a particular malfunction has occurred to the pumping system. This novel concept enables a computer controlled system to provide remedial action for a number of possible well malfunctions in addition to detection and shut-in for fluid pounding.